The present invention generally relates to differential amplifier circuits, and more particularly to isolating multistage differential amplifiers for electrically isolating input and output stages.
Isolating amplifiers are utilized in signal-processing situations in which one portion of a circuit is operated at different voltage levels with respect to another portion of a circuit. For example, an electrical test and measurement instrument may be operated with reference to earth potential while a circuit under test may be floating, or may be elevated to a high voltage potential with respect to ground.
Prior art isolation circuits have relied primarily on transformers to perform the dual role of providing electrical isolation and signal coupling. However, transformers have severe limitations in that they are bulky and are responsive only to changing signals. In the past few years, optical couplers have become popular because of their ability to pass distortion-free signals within their operating limits while simultaneously providing electrical isolation between input and output.
While optical couplers have provided improved low frequency signal coupling, a problem which continues to perplex designers of isolating amplifier circuits is that of gain and offset drift caused by changes either in operating temperature or in the physical parameters of amplifying devices with age. Attempts to solve this problem have included complex compensating networks or feedback circuits.